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Simulink Implementation Simulink Implementation of a Cable Modemof a Cable Modem
Gina ColangeloGina ColangeloNathan EganNathan EganEE194-SDREE194-SDR05/02/200605/02/2006
Topics CoveredTopics Covered
Cable Data Network OverviewCable Data Network Overview
Cable Modem ArchitectureCable Modem Architecture
Downstream/Upstream SpecificationsDownstream/Upstream Specifications
Simulink ImplementationSimulink Implementation
Implementation ResultsImplementation Results
Cable Modem Network OverviewCable Modem Network OverviewHeadend: DOCSIS-certified CMTS (Cable Modem Termination System)Headend: DOCSIS-certified CMTS (Cable Modem Termination System)
One Headend supports ~ 2000 Cable Modem Users on a single TV ChannelOne Headend supports ~ 2000 Cable Modem Users on a single TV Channel CMTS interfaces the CATV network to the InternetCMTS interfaces the CATV network to the Internet
CMTS output channel combined with TV video signalsCMTS output channel combined with TV video signalsCATV Network to Subscriber via coaxial cableCATV Network to Subscriber via coaxial cableOne-to-Two splitter: One signal to Set Top Box (STB), other to Cable ModemOne-to-Two splitter: One signal to Set Top Box (STB), other to Cable ModemCable ModemCable Modem
One Modem can support up to 16 users in a local-area networkOne Modem can support up to 16 users in a local-area networkPC/Ethernet CardPC/Ethernet Card
Cable Modem connected to PC via Ethernet, USB, PCI Bus, etcCable Modem connected to PC via Ethernet, USB, PCI Bus, etc
Cable Modem ArchitectureCable Modem ArchitectureTransmit/UpstreamTransmit/Upstream
QPSK/QAM Modulator performs:QPSK/QAM Modulator performs:QPSK/QAM-16 modulation QPSK/QAM-16 modulation Reed-Solomon EncodingReed-Solomon EncodingD/A ConversionD/A ConversionUp-conversion to the selected Up-conversion to the selected frequency/channelfrequency/channel
Receive/DownstreamReceive/Downstream RF TunerRF Tuner
Converts TV Channel to a fixed Converts TV Channel to a fixed lower frequency (6-40MHz)lower frequency (6-40MHz)
QAM Demodulator performs:QAM Demodulator performs: A/D conversionA/D conversionQAM-64/256 demodulationQAM-64/256 demodulationMPEG frame synchronizationMPEG frame synchronizationError Correction (Reed-Error Correction (Reed-Solomon)Solomon)
MAC - Media Access Control MAC - Media Access Control Implemented partially in Implemented partially in
hardware and softwarehardware and softwareData and Control LogicData and Control Logic
Receive PathReceive PathDiplex Filter – splits/combines bands for 2-way capability on CATV systemsDiplex Filter – splits/combines bands for 2-way capability on CATV systemsTuner – isolates TV channel and mixes it down to Analog IF (6-40MHz)Tuner – isolates TV channel and mixes it down to Analog IF (6-40MHz)Analog to Digital ConversionAnalog to Digital ConversionDecimation Filters to down-sample to the symbol rate (e.g. CIC)Decimation Filters to down-sample to the symbol rate (e.g. CIC)QAM DemodulatorQAM Demodulator
MPEG Frame synchronizationMPEG Frame synchronization Automatic Gain Control (AGC)Automatic Gain Control (AGC) Equalizer – removes distortions, and cancels echoes or multi-path conditionsEqualizer – removes distortions, and cancels echoes or multi-path conditions Carrier RemovalCarrier Removal Automatic Frequency Control (AFC) Automatic Frequency Control (AFC)
QAM Demodulator
Generic Receive Path of a Cable Modem
DiplexFilter
(Internal orExternal to
Modem)
Analog RF
TUNERAnalog IF
ADC
FixedFreq
Decimate to Symbol
Rate
QAMDemodulation
Digital Ctrl Logic/Signals
LNA TunableBPF
Digital Ctrl Logic/Signals
VGA LPF
VariableFreq
Dig BB
Downstream SpecificationsDownstream SpecificationsFrequency Range: 65-850MHzFrequency Range: 65-850MHzBandwidth: 6MHz (USA) or 8MHz (EU)Bandwidth: 6MHz (USA) or 8MHz (EU)Modulation: 64-QAM (6 bits/symbol) or 256-QAM (8 bits/symbol)Modulation: 64-QAM (6 bits/symbol) or 256-QAM (8 bits/symbol)Data rate: 27-56 Mbps (depends on modulation and bandwidth)Data rate: 27-56 Mbps (depends on modulation and bandwidth)
Continuous stream of dataContinuous stream of dataFraming: MPEG-2 (based on DOCSIS spec)Framing: MPEG-2 (based on DOCSIS spec)Encryption: DES (Data Encryption Standard)Encryption: DES (Data Encryption Standard)Reed-Solomon Forward Error CorrectionReed-Solomon Forward Error Correction
* Symbol rate listed under Modulation does not compensate for error correction and other overhead* Symbol rate listed under Modulation does not compensate for error correction and other overhead** Data rates listed in the table use a symbol rate of 6.9 Msym/s for 8MHz BW and 5.2Msym/s for 6MHz BW** Data rates listed in the table use a symbol rate of 6.9 Msym/s for 8MHz BW and 5.2Msym/s for 6MHz BW
64-QAM64-QAM 256-QAM256-QAM
6 MHz6 MHz 31.2 31.2 MbpsMbps
41.6 Mbps41.6 Mbps
8 MHz8 MHz 41.4 41.4 MbpsMbps
55.2 Mbps55.2 Mbps
Transmit PathTransmit PathQAM (Burst) ModulatorQAM (Burst) Modulator
Reed-Solomon Encoder (Forward Error Correction)Reed-Solomon Encoder (Forward Error Correction) Randomizer/Scrambler – whitens the dataRandomizer/Scrambler – whitens the data
Helps to avoid discrete spurs in output spectrumHelps to avoid discrete spurs in output spectrumImproves synchronization at the receiver since data is more equiprobableImproves synchronization at the receiver since data is more equiprobable
Preamble InsertionPreamble InsertionInserts a training sequence into the systemInserts a training sequence into the systemTransmitted without R-S Encoding or ScramblingTransmitted without R-S Encoding or Scrambling
Modulation Encoder (QPSK/16-QAM)Modulation Encoder (QPSK/16-QAM) Pulse Shaping (e.g. Raised Root Cosine)Pulse Shaping (e.g. Raised Root Cosine) Programmable Interpolation Filters (e.g. CIC)Programmable Interpolation Filters (e.g. CIC) Programmable NCO, digitally modulates carrier anywhere in the Nyquist BandwidthProgrammable NCO, digitally modulates carrier anywhere in the Nyquist Bandwidth Digital to Analog ConversionDigital to Analog Conversion
Variable Gain CATV Line DriverVariable Gain CATV Line Driver
Variable GainCATV Line
Driver
DigitalData In
QAM Modulator
Reed-SolomonEncoder
Randomizer
PreambleInsertion
MUX
Pulse-ShapingFIR Filter
Pulse-ShapingFIR Filter
Modulation Encoder
(QPSK/16-QAM))
ProgrammableInterpolation
Filter
ProgrammableInterpolation
Filter
Digital Control Logic/Signals
Program-mableNCO
DAC
FixedFreq
To Diplexer
Upstream SpecificationsUpstream Specifications
Frequency Range: 5-65 MHzFrequency Range: 5-65 MHz
Variable Bandwidth: 200 kHz to 3.2 MHz (2 MHz Variable Bandwidth: 200 kHz to 3.2 MHz (2 MHz typically)typically)
Modulation: QPSK (2 bits/symbol) or 16-QAM (4 Modulation: QPSK (2 bits/symbol) or 16-QAM (4 bits/symbol)bits/symbol)
Data rate: Variable 320 Kbps to 10 MbpsData rate: Variable 320 Kbps to 10 Mbps
Transmit bursts of data in timeslots (TDM)Transmit bursts of data in timeslots (TDM)
Encryption: DESEncryption: DES
Reed Solomon Forward Error CorrectionReed Solomon Forward Error Correction
TX ImplementationTX Implementation
QAM
Modulator
Raised Root Cosine Xmit
Filter
I Xmit FIR
32x
Q Xmit FIR
32x
NCO
I
Q
COSSIN
GAIN
Input Bit Stream
RF OutputSignal
16-QAM Modulation – 320ksps16-QAM Modulation – 320kspsRaised Root Cosine Filter:Raised Root Cosine Filter:
Upsamples by 8, data rate at output = 2.56MspsUpsamples by 8, data rate at output = 2.56Msps Roll-off factor = 0.7Roll-off factor = 0.7
FIR Interpolation FilterFIR Interpolation Filter Polyphase ImplementationPolyphase Implementation Interpolates by 32, data rate at output = 81.92MspsInterpolates by 32, data rate at output = 81.92Msps Order FIR filter = 60, Cutoff Frequency = 320kHzOrder FIR filter = 60, Cutoff Frequency = 320kHz
NCO - Mixes signal up to 30MHzNCO - Mixes signal up to 30MHzGain factor of 30 to compensate for losses throughout pathGain factor of 30 to compensate for losses throughout path
TX SpectrumTX Spectrum
Output signal from up-conversion
1.280
PowerOutput from the RRC Filter:
FIR Interpolation Filter Response
0
PowerSampled signal after interpolation*
40.96-40.96
MHz-1.28
MHz
30-30 0
Power
MHz40.96-40.96
*Interpolation Images will occur every 2.56MHz*Interpolation Images will occur every 2.56MHz
RX ImplementationRX Implementation
QAM
Demodulator
Raised Root Cosine Rx
Filter
I Rx FIR
32x
Q Rx FIR
32x
NCO
I
Q
COSSIN
RF Input
Baseband
Signal
NCO - Mixes signal down to BasebandNCO - Mixes signal down to BasebandFIR Decimation FilterFIR Decimation Filter
Polyphase ImplementationPolyphase Implementation Decimates by 32, data rate at output = 2.56MspsDecimates by 32, data rate at output = 2.56Msps Order FIR filter = 10, Cutoff Frequency = 1.28MHzOrder FIR filter = 10, Cutoff Frequency = 1.28MHz
Raised Root Cosine Filter:Raised Root Cosine Filter: Downsamples by 8, data rate at output = 320kspsDownsamples by 8, data rate at output = 320ksps Roll-off factor = 1 Roll-off factor = 1
16-QAM Demodulation16-QAM Demodulation
RX SpectrumRX SpectrumInput signal to demodulator
0
PowerSampled signal after decimation
600
PowerSignal after the down-conversion Input Signal
0.320-0.32
PowerInput to the QAM demodulator
RRC Filter Response
1.28-1.28
-60
30-30 0
Power
MHz40.96-40.96
MHz
MHz
FIR Decimation Filter Response
MHz
Simulink Block DiagramSimulink Block Diagram
Simulink Results – TX PathSimulink Results – TX Path
TX Signal After Pulse ShapingTX Signal After Pulse Shaping TX Signal After 32X InterpolationTX Signal After 32X Interpolation
TX Signal after Up-ConversionTX Signal after Up-Conversion
Simulink Results – RX PathSimulink Results – RX Path
Rx Signal After Down-Conversion Rx Signal After 32X Decimation
System Results – Time DomainSystem Results – Time Domain
Delay Through Data Path
System Results 2System Results 2
TX Constellation RX Constellation
Simulink Results - BERSimulink Results - BER
BER with current implementation = 0.9741BER with current implementation = 0.9741
What effected BER?What effected BER? Design of the FIRDesign of the FIR SNR of datapath (AWGN Channel)SNR of datapath (AWGN Channel) Design of Raised Root Cosine FilterDesign of Raised Root Cosine Filter
Main TradeoffMain Tradeoff Dynamic Range/Spurious Content vs EVM/BERDynamic Range/Spurious Content vs EVM/BER
Better ImplementationBetter Implementation Compensation Filter after Interpolator and DecimatorCompensation Filter after Interpolator and Decimator
Websites for More Info:Websites for More Info:http://www.ciscopress.com/articles/article.asp?p=31289&seqNum=2&rl=1http://www.ciscopress.com/articles/article.asp?p=31289&seqNum=2&rl=1http://www.iec.org/online/tutorials/acrobat/cable_mod.pdfhttp://www.iec.org/online/tutorials/acrobat/cable_mod.pdfhttp://www.cable-modems.org/tutorial/index.htm#TOChttp://www.cable-modems.org/tutorial/index.htm#TOChttp://www.broadcom.com/collateral/pb/3349-PB01-R.pdfhttp://www.broadcom.com/collateral/pb/3349-PB01-R.pdfhttp://www.analog.com/UploadedFiles/Data_Sheets/13374136AD9853_c.pdhttp://www.analog.com/UploadedFiles/Data_Sheets/13374136AD9853_c.pdffhttp://www.wideband.com/products/Modems/QAM20Mod/QAMmod.htmlhttp://www.wideband.com/products/Modems/QAM20Mod/QAMmod.htmlhttp://focus.ti.com/lit/an/slwa023/slwa023.pdfhttp://focus.ti.com/lit/an/slwa023/slwa023.pdfhttp://www.broadcom.com/collateral/pb/3419-PB02-R.pdfhttp://www.broadcom.com/collateral/pb/3419-PB02-R.pdfhttp://www.cableaml.com/website3/wireless_catv/descriptions.htmhttp://www.cableaml.com/website3/wireless_catv/descriptions.htmhttp://www.fcc.gov/mb/engineering/cablemqa.htmlhttp://www.fcc.gov/mb/engineering/cablemqa.htmlhttp://www.cablemodem.com/http://www.cablemodem.com/http://www.nextgendc.com/index.htmhttp://www.nextgendc.com/index.htmhttp://computer.howstuffworks.com/cable-modem.htmhttp://computer.howstuffworks.com/cable-modem.htmhttp://compnetworking.about.com/od/dslvscablemodem/a/http://compnetworking.about.com/od/dslvscablemodem/a/dslcablecompare.htmdslcablecompare.htm